You are likely not getting enough arch on your turn. When you turn, exaggerate the distance you pull uke's hands from his body, this with better break his balance. If done correctly, he should be falling before you even finnish your throw.
The pulling-back shihonage throw will break uke's arm with a spiral fracture. That's a great idea on the battlefield. It's not such a great idea on the mat, or probably even on the street.